The present invention relates to a voltage/current characteristics control circuit particularly for protecting power transistors.
In power integrated circuits, the need is increasingly felt to adequately protect power transistors, whether in MOS technology or in bipolar technology, from overloads. Power transistors, like all transistors, in fact have, in the voltage/current plane, as more clearly illustrated in FIG. 1, an operating area usually termed SOA, which is the acronym of Safe Operating Area.
The working point of the device must always be within the SOA, under penalty of the destruction of the device itself; an event which push the device outside the SOA obviously requires rapid and drastic maintenance interventions, thus increasing the management costs of the machine or apparatus in which this event may occur.
The boundaries of the SOA are usually defined by three limits: the maximum current I.sub.MAX which can be withstood by the output device, its maximum voltage V.sub.MAX and the maximum power which can be dissipated by the device.
In the case of devices, the situation is further limited by the well-known phenomenon of II breakdown, indicated in FIG. 1 by the broken line, which is ultimately the main restriction in the manufacture of bipolar power devices for high voltages, i.e., with a supply voltage V.sub.cc higher than 30 volts.
Furthermore, in particular conditions, such as an amplifier with a partially inductive load, the SOA can rather often be abandoned by the locus of the points of the voltage/current plane which represent the work point region, as more clearly illustrated in FIG. 2. This boundary crossing unavoidably entails the final destruction of the device.
The technical problem therefore consists in manufacturing protection circuits which detect when the power transistor is going to cross the boundaries of the SOA and switch it off in this case.
The situation is even worse when the difference between the boundary of the SOA and the work point is very small; the protection circuits must therefore have such a resolution as to follow the SOA as closely as possible, so as to avoid switching the transistor off when instead it is still within the SOA, i.e., in normal operating conditions.
Finally, the manufacture of said protection circuits and, most of all, their operating stability with respect to variations of the production processes for their manufacture is one of the main problems of protection circuits.